In real applications, especially automotive systems, an integrated circuits (IC) voltage supply, ground or even signal lines could be accidently connected to an unregulated power supply or most negative voltage in the system, resulting in permanent damage to the internal circuits by forward-biasing those ESD or parasitic diodes. Those fault conditions include overvoltage, reverse voltage and under-voltage.
For an overvoltage fault conditions, a conventional solution is to put a high voltage (HV)MOSFET between an external voltage supply and the internal circuits. The gate of this HV MOSFET is either regulated so that the drain of the HV MOSFET outputs a fixed value or is turned off when overvoltage happens. The HV MOSFET typically cannot handle reverse voltage condition because of the parasitic diode between drain and bulk of the HV MOSFET.
For reverse voltage fault conditions, there are three main methods: (1) place a specific diode in series between external supply and internal circuits, this method reduces the voltage headroom for the internal circuits, plus provides power dissipation. (2) use a floating-well device, this method is cost-effective and power-efficient, but it is very susceptible to latch-up and noise and (3) place two MOSFET in series with this bulk connected to source and drain separately, the gate control logic of this tri-state conventionally and also consumes a large current.
Methods focus on the supply line fault connections and use the above identified overvoltage and reverse voltage concepts to protect the internal circuits. In the automotive applications, the signal lines could also have such fault connections. The conventional solutions to this signal line protection is to provide series resistor to limit the maximum current into/out IC. Using this series resistor is not desirable for applications in which the tri-state output is required during fault conditions. The value of this series resistor in some instances has to be larger than 100 Ohm and have limited application.
Hence, conventional systems have the following shortcomings:
Reduced voltage headroom for internal circuits.
Susceptible to latch-up and noise.
Introduce big output impedance on signal lines.
Cannot provide tri-state for certain applications.
Accordingly, what is desired is to provide a system and method that overcomes the above issues. The present invention addresses such a need.